The present invention relates to a process for producing electronic devices incorporating a layer of silicon or one of its alloys, more particularly, photovoltaic cells usable for converting photon radiation into electric energy.
Photovoltaic cells generally incorporate a multilayer structure, for example, a p-n junction or a metal-semiconductor contact. Silicon is generally used as the material. In its monocrystalline or polycrystalline form with macroscopic grains of sizes exceeding 10.sup.-5 cm, silicon absorbs little light, so that the minimum thickness of the layer to be deposited for obtaining an appropriate efficiency is roughly a few dozen microns. To reduce the cost of cells it is desirable to use the thinnest possible layers, of the order of one micron. For this purpose it is possible to use amorphous silicon, whose optical absorption is well above that of crystalline silicon. However, amorphous silicon obtained, for example, by low temperature decomposition of silane, generally has inferior electrical characteristics to crystalline silicon. For example, the mobility of the electrons is approximately 0.1 cm.sup.-2 V.sup.-1 s.sup.-1, compared with that of crystalline silicon, which can be approximately 10.sup.-3 cm.sup.-2 V.sup.-1 s.sup.-1.
French patent application No. 77 17 245, filed by the present assignee and published as No. 2 394 173, proposes a process for producing devices incorporating an amorphous silicon layer, making it possible to improve the electrical properties of the layer. This process consists of depositing the amorphous silicon layer by evaporation or atomization in vacuo or at reduced pressure and at a low temperature. This is followed by a heat treatment of the deposited layer in a hydrogen plasma or in a plasma of one of its isotopes at a relatively low temperature, so that the silicon does not crystallize. The hydrogenation process applied to the amorphous silicon makes it possible to control the hydrogen proportion contained in the material, which is not easy in the case of silane decomposition in a glow discharge. The advantages obtained are mainly based on flexibility of the preparation mode, but the better electrical characteristics obtained remain similar. However, when a doped silicon layer is required the deposition method used does not make it possible to bring about an easy introduction of the dopant.
It has been found that by using a deposition method involving thermal dissociation at high temperature, i.e. close to the crystallization temperature of the deposited material (pure or doped silicon) after hydrogenation the results obtained are much better than those obtained with the known processes. At this temperature the layer has a hybrid structure between the amorphous form and a polycrystalline form with very fine grains. After treatment in a hydrogen plasma the conductivity values obtained are closer to the corresponding values for monocrystals and the optical absorption values are virtually as high as those obtained by the prior art process, i.e. the values obtained with amorphous layers.